1. Field of the Invention
This invention relates to a method of producing a thermoplastic polymer film utilizing the electrostatic application method.
2. Description of the Prior Art
The electrostatic application method comprises, in a manufacturing process of a film by the extrusion of a melted thermoplastic polymer, providing a wire or plate electrode between a die and a rotary cooling body, depositing an electrostatic charge on the surface of the film in a melted state by using the rotary cooling body as the counter electrode, and allowing the film to adhere uniformly on the surface of the rotary cooling body to thereby cool the film rapidly. The electrostatic application method is disclosed in U.S. Pat. No. 3,223,757, Japanese Patent KOKOKU Nos. 48-3535, 48-14786, 48-29311 and the like.
The electrostatic application method is excellent for preventing the slip of the film and the entrapment of air, and for reducing the shrinkage of the film in the cross direction caused by the rapid cooling. However, when the production speed is elevated in order to raise the productivity, the electrostatic charge deposited per an unit area of the film on the surface of the rotary cooling body decreases according to the increase of the circumferential speed of the cooling body. As a result, the adhesive force is decreased, and air is trapped between the film and the surface of the cooling body. In such a state, the efficiency of heat transfer between the film and the rotary cooling body is decreased by the tapped air layer, and the flatness of the film is damaged by the trapped air bubbles between the film and the rotary cooling body. When the impressed voltage to the wire electrode is elevated in order to increase the adhesive force, a sufficient electrostatic application cannot be achieved. Particularly, there is an upper limit in the impressed voltage due to the generation of spark discharge between the wire and the rotary cooling body or the film. Moreover, the spark discharge between the wire and the rotary cooling body results in scarring of the surface of the rotary cooling body, and the spark discharge between the wire and the film generates pinholes on the film.
In order to compensate such a disadvantage of the electrostatic application method, various proposals have been made. For example, Japanese Patent KOKAI No. 53-14762 discloses a method for making uniform the turbulence of the electric field caused by the unevenness of a die surface by providing a shield composed of a laminate of a metal layer and an insulating layer between a wire electrode and a die surface faced thereto so that the insulating layer is disposed on the side of the wire electrode. Japanese Patent KOKAI No. 58-163624 discloses a method of extending the life of a wire electrode by providing a second electrode close to the wire electrode on the side opposite to the rotary cooling body, and impressing a voltage simultaneously on the second electrode and the wire electrode as the same kind of electrode. U.S. Pat. No. 4,650,411 discloses a method of inhibiting neck-in effect by providing a brush electrode on both edge portions of the rotary cooling body so as to face the cooling body. Japanese Patent KOKOKU No. 61-19415 discloses a method of improving the adhesion of the extruded film to the rotary cooling body by using a grounded auxiliary electrode, in addition to the main electrode of a metal wire, composed of a conductive material being planar or having an opening provided on the side opposite to the rotary cooling body. Japanese Patent KOKAI No. 51-146560 discloses a method of depositing the same kind of an electric charge on the surface of a thermoplastic polymer on the side not contacting the cooling body and on the surface provided with an insulating coating of a rotating roll disposed on the same side.
However, the above methods are still insufficient in view of increasing the density of electric charge and raising the directivity of the electric charge toward the extruded film for improving the adhesion of film in the electrostatic application method.